The present invention in general relates to a receiver by which a television broadcast signal and an FM radio broadcast signal can be selectively received and can be demodulated and output. More particularly, this invention relates to a receiver which is lightweight and small in size.
Recently, accompanied with the prevalence of portable color televisions, for example liquid crystal televisions, a television receiver receiving a general FM broadcast has appeared. This television receiver can receive sound signals of FM radio broadcast signals by employing a high frequency circuit, such as a tuner, and a sound signal demodulation circuit for a television broadcast signal. When a sound signal of an FM radio broadcast signal is demodulated, the demodulation does not employ a video intermediate carrier signal of a television broadcast signal.
FIG. 6 is a block diagram showing the structure of a conventional television receiver which can also receive an FM radio broadcast signal. First, the occasion on which a television broadcast signal is received is explained. A tuner 112 is used for tuning to a television broadcast signal received via an antenna 111. The tuner 112 down converts the received signal to an intermediate frequency, and outputs this intermediate frequency band signal to SAW filters 113, 121. The SAW filter 113 passes the intermediate frequency band having a video intermediate carrier frequency, fo, and outputs it to a VIF amplifier 114. The VIF amplifier 114 amplifies the intermediate frequency band signal and outputs it to a video detector 116 and also to a switch 120.
The switch 120 is connected to a terminal to which the output of the VIF amplifier 114 is supplied when receiving the television broadcast signal, and the intermediate frequency band signal is output to an APC (automatic phase control circuit) 118 via the switch 120. The APC 118 detects the phase difference between (i) the video intermediate carrier frequency fo in the intermediate frequency band signals and (ii) a signal that a voltage control oscillation circuit (VCO) 117 outputs, and outputs the phase difference to the VCO 117. The VCO 117 outputs the signal of the video intermediate carrier frequency fo which is phase locked to the video intermediate carrier frequency fo to the video detector 116 based on the phase difference. The video detector 116 detects a video signal based on the signal of the video intermediate carrier frequency fo that the VCO 117 outputs and outputs the detected video signal to a terminal 119. The detection output of the video detector 116 is outputted to a VIFAGC circuit 115, and the VIFAGC circuit 115 provides gain control to stabilize the amplitude of the detection output of the video detector 116 with respect to the VIF amplifier 114.
On the other hand, the SAW filter 121 passes an intermediate frequency (fo-fs), and a QIF amplifier 122 amplifies this intermediate frequency based on the gain control by a QIFAGC circuit 123 and outputs it to an intermediate frequency detection circuit (QIFDET) 124. The intermediate frequency detection circuit 124 mixes the inputted intermediate frequency (fo-fs) with the video intermediate carrier frequency fo output from the VCO 117 and outputs a sound subcarrier frequency, fs. Then, the sound subcarrier frequency fs is extracted via a band pass filter 131 and is amplified by an SIF amplifier 132, and a sound signal detected and output by an FM detector 133 is output from a terminal 134.
When receiving the FM radio broadcast signal, the tuner 112 down converts the signal to an intermediate frequency that is the same as the intermediate frequency (fo-fs) and outputs it to the SAW filter 121. After that, the FM sound intermediate frequency fs is detected by the intermediate frequency detection circuit 124, similarly to the sound subcarrier frequency fs, and, lastly, a sound signal is output from the terminal 134. In this case, since the VCO does not output the video intermediate carrier frequency fo, the switch 120 is connected to the input side of the oscillation frequency fo corresponding to the video intermediate carrier frequency fo, and the oscillation frequency fo is output to the APC 118. In this way, the VCO 117 outputs a phase locked oscillation frequency fo to the intermediate frequency detection circuit 124 and detects and outputs the FM sound intermediate frequency fs.
In the conventional receiver, a video signal related intermediate frequency processing circuit of a television broadcast signal and a sound signal related intermediate frequency processing circuit of a sound signal of a television broadcast signal and a sound signal of an FM radio broadcast signal are independently provided. Accordingly, demodulation characteristic of a sound signal of an FM radio broadcast signal can be set optimally.
However, since the video signal related intermediate frequency processing circuit and the sound signal related intermediate frequency processing circuit are independently provided, many parts and circuit blocks have to be provided, and thus there is a problem that the cost increases and also it is impossible to obtain a lightweight and small size receiver.
It is an object of the present invention to obtain a receiver by which an FM radio broadcast signal can be received in addition to the reception of a television broadcast receiving signal and which is lightweight and small in size.
In the receiver according to one aspect of this invention, when receiving the television broadcast signal, a first switch selects a video intermediate carrier signal output by an intermediate frequency amplifier and a second switch is switched towards and connected to a first automatic gain controller. A tuner converts the television broadcast signal into an intermediate frequency, and a band limit filter passes this intermediate frequency so that the intermediate frequency is amplified by the intermediate frequency amplifier. On this occasion, the intermediate frequency amplifier is gain controlled so that the amplitude of the video signal that the video detector outputs is made constant by the first automatic gain controller. When receiving the FM radio broadcast, the first switch is switched towards a self-running oscillation signal, and the second switch is switched towards and connected to a second automatic gain controller. The tuner converts the FM radio broadcast signal into an intermediate frequency that is the same as the sound intermediate frequency subcarrier signal when receiving the television broadcast signal and outputs it when receiving the FM radio broadcast signal. The band limit filter passes this intermediate frequency, and the intermediate frequency is amplified by the intermediate frequency amplifier. On this occasion, the intermediate frequency amplifier is gain controlled so that the amplitude of the sound subcarrier signal that the sound band limit filter outputs is made constant by the second automatic gain controller. With this, the band limit filter of the exclusive use of sound, the intermediate frequency amplification circuit of the exclusive use of sound, and the intermediate frequency detection circuit of the exclusive use of sound become unnecessary, and a sound signal having a high quality can be demodulated and outputted even when receiving the FM radio broadcast signal.
In the receiver according to another aspect of this invention, when receiving the television broadcast signal, a first switch selects a video intermediate carrier signal output by an intermediate frequency amplifier, and a second switch is switched towards and connected to a first automatic gain controller. A tuner converts the television broadcast signal into an intermediate frequency, and a band limit filter passes this intermediate frequency so that the intermediate frequency is amplified by the intermediate frequency amplifier. On this occasion, the intermediate frequency amplifier is gain controlled so that the amplitude of the video signal that the video detector outputs is made constant by the first automatic gain controller. After that, the video detector outputs the video signal containing the sound subcarrier signal from the intermediate frequency signal of the video intermediate frequency band amplified by the intermediate frequency amplifier. When receiving the FM radio broadcast, the first switch is switched towards a self-running oscillation signal, and the second switch is switched towards and connected to a second automatic gain controller. The tuner converts the FM radio broadcast signal into an intermediate frequency that is the same as the sound intermediate frequency subcarrier signal when receiving the television broadcast signal and outputs it when receiving the FM radio broadcast signal. The band limit filter passes this intermediate frequency, and the intermediate frequency is amplified by the intermediate frequency amplifier. On this occasion, the intermediate frequency amplifier is gain controlled so that the amplitude of the sound subcarrier signal that the sound band limit filter outputs is made constant by the second automatic gain controller. After this, the video detector detects and outputs the sound subcarrier signal from the intermediate frequency signal of the video intermediate frequency band amplified by the intermediate frequency amplifier. On this occasion, the video detector detects and outputs the sound subcarrier signal based on the self-running oscillation signal that a second phase synchronizer outputs which is provided independently of a first phase synchronizer employed when receiving the television broadcast signal. With this, the band limit filter of the exclusive use of sound, the intermediate frequency amplification circuit of the exclusive use of sound, and the intermediate frequency detection circuit of the exclusive use of sound become unnecessary. Further, detecting and outputting a frequency is performed by the first phase synchronizer and the second phase synchronizer each provided independently so that quality deterioration of the sound signal due to unevenness in phase synchronization when receiving the FM radio broadcast signal can be restrained.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.